On the ST-70 you can check the 7-digit number stamped on the bottom of the BIAS POTENTIOMETERS. The 4th and 5th digits are the year of manufacture and the 6th and 7th numbers are what week of that year. If for example the bias pot is numbered 1386217 - then these bias pots were made in the 17th week of 1962 - probably in early May of 1962. Since Dynaco used parts very quickly you can almost be assured that this amp kit was shipped to a dealer in 1962. If your bias pot has a "48 to 52" as the 6th and 7th numbers like "1386249" then it is possible that the pot was made in 1962 but the amp was shipped out 5 or 6 weeks later in 1963.
See below a bias pot made in August of 1964 with a "1376430" code